PSR J0737−3039 is the only known double pulsar. It consists of two neutron stars emitting electromagnetic waves in the radio wavelength in a relativistic binary system. The two pulsars are known as PSR J0737−3039A and PSR J0737−3039B. It was discovered in 2003 at Australia's Parkes Observatory by an international team led by the radio astronomer Marta Burgay during a high-latitude pulsar survey.
Pulsars
A pulsar is a neutron star which produces pulsating radio emission due to a strong magnetic field. A neutron star is the ultra-compact remnant of a massive star which exploded as a supernova. Neutron stars have a mass bigger than our sun, yet are only a few kilometers across. These extremely dense objects rotate on their axes, producing focused electromagnetic waves which sweep around the sky in a lighthouse effect at rates that can reach a few hundred pulses per second. PSR J0737−3039 is the only known system containing two pulsars - thus a 'double pulsar' system. The object is similar to PSR B1913+16, which was discovered in 1974 by Taylor and Hulse, and for which the two won the 1993 Nobel Prize in Physics. Objects of this kind enable precise testing of Einstein'stheory of general relativity, because the precise and consistent timing of the pulsar pulses allows relativistic effects to be seen when they would otherwise be too small. While many known pulsars have a binary companion, and many of those are believed to be neutron stars, J0737−3039 is the first case where both components are known to be not just neutron stars but pulsars.
Physical characteristics
The orbital period of J0737−3039 is the shortest yet known for such an object, which enables the most precise tests yet. In 2005, it was announced that measurements had shown an excellent agreement between general relativity theory and observation. In particular, the predictions for energy loss due to gravitational waves appear to match the theory. As a result of energy loss due to gravitational waves, the common orbit shrinks by 7 mm per day. The two components will coalesce in about 85 million years. Due to relativistic spin precession, the pulses from Pulsar B are no longer detectable as of March 2008, but are expected to reappear in 2035 due to precession back into view.
Discovery
PSR J0737−3039A was discovered in 2003, along with its partner, at Australia's 65 m antenna of the Parkes Radio Observatory; J0737−3039B was not identified as a pulsar until a second observation. The system was originally observed by an international team during a high-latitude multibeam survey organized in order to discover more pulsars in the night sky. Initially this star system was thought to be an ordinary pulsar detection. The first detection showed one pulsar with a period of 23 milliseconds in orbit around a neutron star. Only after follow up observations was a weaker second pulsar detected with a pulse of 2.8 seconds from the companion star. Although over 1400 pulsars have been detected since their discovery in 1967 by Anthony Hewish and Jocelyn Bell at Cambridge University, this particular system has caused a lot of excitement. Previous observations have recorded a pulsar orbiting a neutron star, but never two pulsars orbiting each other.
Implications
The double pulsar system PSR J0737−3039 is being studied in order to test Einstein's general theory of relativity put forward in 1915. The investigation of double pulsars is a great opportunity as the environment created by warped space-time due to the shift of intense masses is extremely rare, and thus perfect for the testing of Einstein's theory and the observation of gravitational waves.
Unique origin
In addition to the importance of this system to tests of general relativity, Piran and Shaviv have shown that the young pulsar in this system must have been born with no mass ejection, implying a new process of neutron star formation that does not involve a supernova. Whereas the standard supernova model predicts that the system will have a proper motion of more than hundred km/s, they predicted that this system would not show any significant proper motion. Their prediction was later confirmed by pulsar timing.
Eclipses
Another great discovery from the double pulsar is the observation of an eclipse from a conjunction of the superior and weaker pulsar. This happens when the doughnut shaped magnetosphere of one pulsar, which is filled with absorbing plasma, blocks the companion pulsar's light. The blockage, lasting more than 30 s, is not complete, due to the orientation of the plane of rotation of the binary system relative to Earth and the limited size of the weaker pulsar's magnetosphere; some of the stronger pulsar's light can still be detected during the eclipse.
Other binary systems
A whole range of differing two-body systems can occur, where a pulsar exists. Other than a double pulsar system, these systems also occur: A pulsar has recently been detected very near the super-massive black hole at the core of our galaxy, but its motion has not yet been officially confirmed as a capture orbit of Sgr A*. A pulsar–black hole system could be an even stronger test of Einstein's theory of general relativity, due to the immense gravitational forces exerted by both celestial objects. The Square Kilometre Array, a planned radio telescope due to be constructed in the southern hemisphere in 2018, will observe binary pulsar systems. It will also search for pulsar-black hole systems in order to test general relativity.
